Fiber optic networks are being deployed at an ever-increasing rate due, at least in part, to the large bandwidth provided by fiber optic cables. Inherent with any fiber optic network design is the need to connect individual optical fibers to other optical fibers and to equipment. A common technique for connecting optical fibers is by terminating an optical fiber with a ferrule, and bringing the ferrule into a mating relationship with another ferrule terminating a second optical fiber. This type of connection is referred to as a ferrule connection. Examples of ferrule connection systems can be found in U.S. Pat. Nos. 4,738,507 and 4,738,508, both issued to Palmquist and assigned to the assignee of the present invention.
When two optical fibers are connected to one another, such as by a ferrule connection, there exists a potential for loss of optical power due to an imperfect transfer of the optical signal from one optical fiber to the other. The loss can be attributed to a number of different factors, but is most commonly caused by a lateral, or transverse, offset in the co-axial alignment of the passageways defined by the ferrules, hereinafter referred to as the cores. In fact, aligning the cores of two ferrules is a formidable task due to the extremely small size of the cores, which can be as small as 8-9 micrometer (.mu.m) in diameter. To further complicate the task of co-axially aligning the cores with exacting precision is the fact that the core of a ferrule is typically slightly offset from the center of the ferrule as a result of the manufacturing process of the ferrule. This offset is often referred to as the concentricity (also referred to as eccentricity) of the core with respect to the ferrule, and includes an angular component and a magnitude component. The angular component provides the direction of the offset, while the magnitude component provides an absolute distance between the ferrule center and the core center.
Accordingly, in connecting two optical fibers together, a coupling device, which typically includes a sleeve, is utilized to co-axially align the cores by engaging the alignment surfaces of the respective ferrules. The sleeve is generally a rigid cylindrical structure, as described in U.S. Pat. No. 4,378,508, cited hereinbefore. However, the great disparity in the size of the ferrule and the core makes the alignment of the cores difficult. As previously mentioned, the core may be as small as 8-9 .mu.m in diameter, whereas the ferrule may be 1,250-2,500 .mu.m in diameter. Moreover, the misalignment of the cores may be caused by the concentricity error of the respective cores, which may reduce the amount of light energy transmitted between the two optical fibers. For example, if the two cores being connected are offset in opposite directions, then the core overlap will be minimized when the ferrules are placed end-to-end. It is desirable to align the cores within 1 .mu.m, though certain applications require the core be aligned within 0.3 .mu.m.
A solution to the offset problem is the tunable connector. A tunable connector is constructed so that the ferrule can be rotated within the ferrule housing and secured in more than one position, typically in three or more positions. For instance, the LC.TM. fiber optic connector manufactured by Lucent Technologies, Inc., USA, is tunable into six equally spaced positions that are approximately 60.degree. apart. Alternatively, the SC connector is tunable into four equally spaced positions, approximately 90.degree. apart. Thus, the offset of the core center with respect to the ferrule center is determined, and then the ferrule is adjusted so that the offset is in a predetermined direction. Therefore, the cores are offset in substantially the same radial plane with respect to the longitudinal axis of the cores, which may increase the overlap between the two cores. However, tunable connectors require precise and reliable methods for determining the offset so that the connectors are accurately tuned.
Several methods have been proposed for accurately measuring the offset of a core with respect to a ferrule center, a few of which are discussed below.
The first method involves viewing the core under a high-powered microscope while the ferrule is rotating within a fixture, such as a V-shaped support block. The movement of the core is measured as the ferrule is rotated about its longitudinal axis. The locus of points defining the center of the core is, in general, circular as the ferrule is rotated, and the radius of the circle is equal to the concentricity error. The aforementioned technique is described in more detail in U.S. Pat. No. 4,738,508, cited hereinbefore.
A second method focuses upon measuring the effect of the offset and involves interconnecting the ferrule under test to a reference connector, sometimes referred to as a "golden connector," which is known to have a negligible concentricity error. After establishing the connection with a coupling structure, the light transmission there through is measured. The offset is determined based upon the loss of light and one or more mathematical equations that define the light loss as a function of the offset.
A third method for measuring the concentricity error involves the imaging of the core and several boundary or edge segments of the ferrule end face. A center for the ferrule and a center for the core are determined based upon the acquired images, and the core offset is determined therefrom, as described in greater detail in U.S. Pat. No. 5,729,622 issued to Cspikes et al. All-optical methods of measuring concentricity such as the one suggested in Cspikes et al. generally have been difficult to implement because of the large disparity between the size of the core and that of the ferrule (usually at least two orders of magnitude).
While the aforementioned methods have some merit, several require physical movement of the ferrule which causes undesirable wear of the testing equipment, several are operator dependent and labor-intensive, yet others are computationally intensive, and require expensive equipment. Accordingly, a heretofore unsatisfied need exists in the industry for a system and method of precisely measuring the concentricity error of an optical fiber ferrule that is less labor-intensive, less computationally intensive, less expensive, and more reliable than presently known systems.